For my elucidation, why? The indentation of the code isn't ideal but
the whole purpose of dynamic-wind is to handle code leaving the thunk in
case of exception, application of a continuation object or regular
return.
You're right about the second and third cases, but not the first. Raising an exception *simply* calls the current exception handler, which is then free to return to the point of call like any other procedure. In such a case, the after-thunk is not run, since dynamically you are still inside the main thunk. This is a fundamental distinction between Lisp exceptions and every other language's exceptions.
Only if the exception handler uses call/cc to escape back to the dynamic context where the handler was set up is the after-thunk run. This is done for you automatically if you use `handle-exceptions` or `chicken-case` or R7RS `guard`.
If you raise an exception with `abort` or `error`, then if the handler returns, another exception "unexpected return from handler" is raised.
Note also that when the thread scheduler switches to a new thread, any before- or after-thunks are not run, as the thread is supposed to be unaware that the switch happened.